Fiber gratings are incorporated into components that form the backbone of modern information and communications technologies, and are suitable for a wide range of applications, such as information processing and optical fiber communication systems utilizing wavelength division multiplexing (WDM). There are many different fiber grating types and configurations. For example, fiber Bragg gratings are useful in lasing, filtering and sensing applications. Various Bragg grating configurations also include chirped fiber gratings useful in chromatic dispersion compensators and apodized fiber gratings that are used to eliminate sidelobes in signal transmission spectra.
Another type of fiber grating—a long period grating—is of particular interest in sensing and filtering applications. Light passing through a long period grating is modified rather than reflected, as occurs in fiber Bragg gratings. Also, unlike a fiber Bragg grating, a long period grating is typically used for coupling the mode of the fiber core into the fiber cladding. A long period grating has a spectral characteristic with multiple transmission gaps. The positions of these gaps along the spectral range depend on the refractive index of a medium outside the cladding of the fiber. Thus, changing the outside refractive index produces a shift in the transmission gaps. Typically, the period of a long period grating is significantly longer than the wavelength of light passing through the grating.
Utilizing novel techniques disclosed in a number of commonly assigned and patents and co-pending patent applications, all of which are incorporated herein by reference in their entirety single helix long period gratings may be fabricated by twisting a fiber with a circular off-centered core. The single helix configuration is advantageous for sensing and gain-flattening applications because of its robustness, low cost and polarization insensitivity. However, in practice, it is difficult to create and maintain the core offset to obtain desirable long period grating characteristics. In addition, increasing the core offset results in excessive coupling losses to conventional, concentric fibers. Moreover, selecting a particular helix diameter for the resulting grating requires design and pre-fabrication of a perform with a particular core offset value. A change in the desired helix diameter requires preparation of an entirely new perform configuration. Additionally, fiber structures with non-concentric cores are difficult to splice with conventional fibers with concentric cores.
It would thus be desirable to provide a single helix fiber grating and a method of fabrication thereof that addresses all of the above challenges.